Estradiol alters body temperature regulation in the female mouse

Sleep disturbance associated with the menopause

IMPORTANCE AND OBJECTIVES

Sleep disturbance is one of the most common and debilitating symptoms experienced by women during the menopause transition. However, there are currently no therapies specifically approved for sleep disturbance associated with the menopause. Here, we consider how to characterize sleep disturbance associated with the menopause and discuss its etiology, including the latest advances in our understanding of the neuronal circuits that regulate reproduction, body temperature, sleep, and mood; and reflect on its impact on women's health and well-being. We also examine the current treatment landscape and look to the future of treatment for this condition.

METHODS

We conducted a review of the literature and combined this with discussion with experts in the fields of sleep and menopause as well as experiences from our own clinical practices.

DISCUSSION AND CONCLUSIONS

Sleep disturbance associated with the menopause is characterized by frequent night-time awakenings and increased awake time after sleep onset. Its impacts are wide-ranging, negatively affecting health as well as personal and social relationships, productivity, and work performance. There is currently an unmet need for effective, safe, and well-tolerated treatments to address this important symptom, and wider recognition of the association between sleep disturbances and the menopause is needed. Sleep disturbances associated with the menopause can result from hormone changes as well as vasomotor and mood symptoms. Growing research has contributed to our knowledge of the role of hypothalamic estrogen-sensitive kisspeptin/neurokinin B/dynorphin neurons. These neurons are thought to integrate the gonadotropin-releasing hormone pathway and the pathways responsible for the homeostatic control of body temperature and the circadian regulation of sleep-wake cycles. Understanding these neurons offers the potential to create treatments that target a key cause of sleep disturbance associated with the menopause. Further research to understand their etiology and characterize the neuronal circuits responsible could benefit the development of these targeted treatment approaches.

The Emerging Therapeutic Potential of Kisspeptin and Neurokinin B

Kisspeptin (KP) and neurokinin B (NKB) are neuropeptides that govern the reproductive endocrine axis through regulating hypothalamic gonadotropin-releasing hormone (GnRH) neuronal activity and pulsatile GnRH secretion. Their critical role in reproductive health was first identified after inactivating variants in genes encoding for KP or NKB signaling were shown to result in congenital hypogonadotropic hypogonadism and a failure of pubertal development. Over the past 2 decades since their discovery, a wealth of evidence from both basic and translational research has laid the foundation for potential therapeutic applications. Beyond KP's function in the hypothalamus, it is also expressed in the placenta, liver, pancreas, adipose tissue, bone, and limbic regions, giving rise to several avenues of research for use in the diagnosis and treatment of pregnancy, metabolic, liver, bone, and behavioral disorders. The role played by NKB in stimulating the hypothalamic thermoregulatory center to mediate menopausal hot flashes has led to the development of medications that antagonize its action as a novel nonsteroidal therapeutic agent for this indication. Furthermore, the ability of NKB antagonism to partially suppress (but not abolish) the reproductive endocrine axis has supported its potential use for the treatment of various reproductive disorders including polycystic ovary syndrome, uterine fibroids, and endometriosis. This review will provide a comprehensive up-to-date overview of the preclinical and clinical data that have paved the way for the development of diagnostic and therapeutic applications of KP and NKB.

New advances in menopause symptom management

Vasomotor symptoms (VMS) are characteristic of menopause experienced by over 75% of postmenopausal women with significant health and socioeconomic implications. Although the average duration of symptoms is seven years, 10% of women experience symptoms for more than a decade. Although menopausal hormone therapy (MHT) remains an efficacious and cost-effective treatment, its use may not be suitable in all women, such as those at an increased risk of breast cancer or gynaecological malignancy. The neurokinin B (NKB) signaling pathway, together with its intricate connection to the median preoptic nucleus (MnPO), has been postulated to provide integrated reproductive and thermoregulatory responses, with a central role in mediating postmenopausal VMS. This review describes the physiological hypothalamo-pituitary-ovary (HPO) axis, and subsequently the neuroendocrine changes that occur with menopause using evidence derived from animal and human studies. Finally, data from the latest clinical trials using novel therapeutic agents that antagonise NKB signaling are reviewed.

Invited review: Translating kisspeptin and neurokinin B biology into new therapies for reproductive health

The reproductive neuropeptide kisspeptin has emerged as the master regulator of mammalian reproduction due to its key roles in the initiation of puberty and the control of fertility. Alongside the tachykinin neurokinin B and the endogenous opioid dynorphin, these peptides are central to the hormonal control of reproduction. Building on the expanding body of experimental animal models, interest has flourished with human studies revealing that kisspeptin administration stimulates physiological reproductive hormone secretion in both healthy men and women, as well as patients with common reproductive disorders. In addition, emerging therapeutic roles based on neurokinin B for the management of menopausal flushing, endometriosis and uterine fibroids are increasingly recognised. In this review, we focus on kisspeptin and neurokinin B and their potential application as novel clinical strategies for the management of reproductive disorders.

Deletion of Growth Hormone Secretagogue Receptor in Kisspeptin Neurons in Female Mice Blocks Diet-Induced Obesity

The gut peptide, ghrelin, mediates energy homeostasis and reproduction by acting through its receptor, growth hormone secretagogue receptor (GHSR), expressed in hypothalamic neurons in the arcuate (ARC). We have shown 17β-estradiol (E2) increases Ghsr expression in Kisspeptin/Neurokinin B/Dynorphin (KNDy) neurons, enhancing sensitivity to ghrelin. We hypothesized that E2-induced Ghsr expression augments KNDy sensitivity in a fasting state by elevating ghrelin to disrupt energy expenditure in females. We produced a Kiss1-GHSR knockout to determine the role of GHSR in ARC KNDy neurons. We found that changes in ARC gene expression with estradiol benzoate (EB) treatment were abrogated by the deletion of GHSR and ghrelin abolished these differences. We also observed changes in metabolism and fasting glucose levels. Additionally, knockouts were resistant to body weight gain on a high fat diet (HFD). Behaviorally, we found that knockouts on HFD exhibited reduced anxiety-like behavior. Furthermore, knockouts did not refeed to the same extent as controls after a 24 h fast. Finally, in response to cold stress, knockout females had elevated metabolic parameters compared to controls. These data indicate GHSR in Kiss1 neurons modulate ARC gene expression, metabolism, glucose homeostasis, behavior, and thermoregulation, illustrating a novel mechanism for E2 and ghrelin to control Kiss1 neurons.

DIFFERENTIAL CONTRIBUTION OF ESTROGEN RECEPTORS TO THE INTESTINAL THERAPEUTIC EFFECTS OF 17β-ESTRADIOL IN A MURINE MODEL OF PARKINSON'S DISEASE

Beneficial effects of estrogens have been reported in Parkinson's disease (PD) for many years. We previously reported their neuroprotective and anti-inflammatory potentials in the enteric nervous system of the intestine, a region possibly affected during the early stages of the disease according to Braak's hypothesis. Three different estrogen receptors have been characterized to date: the estrogen receptor alpha (ERα), the estrogen receptor beta (ERβ) and the G protein coupled estrogen receptor 1 (GPER1). The aim of the present study was to decipher the individual contribution of each estrogen receptor to the therapeutic properties of 17β-estradiol (E2) in the myenteric plexus of the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of PD. Different agonists, 4,4',4''-(4-propyl-[1H]-pyrazole-1,3,5-triyl)trisphenol (PPT; ERα), 2,3-bis(4-hydroxyphenyl)-propionitrile (DPN; ERβ), G1 (GPER1), and antagonists, ICI 182,780 (ERα and ERβ), G15 (GPER1), were used to analyze the involvement of each receptor. We confirmed that G1 protects dopamine (DA) neurons to a similar extent as E2. An anti-inflammatory effect on proinflammatory macrophages and cultured human monocytes was also demonstrated with E2 and G1. The effects of PPT and DPN were less potent than G1 with only a partial neuroprotection of DA neurons by PPT and a partial reduction of interleukin (IL)-1β production in monocytes by PPT and DPN. Overall, the present results indicate that the positive outcomes of estrogens are mainly through activation of GPER1. Therefore, this suggests that targeting GPER1 could be a promising approach for future estrogen-based hormone therapies during early PD.

Investigation of the Effects of Cardiovascular Therapeutic Ultrasound Applied in Female and Male Rats' Hearts of Different Ages

This study investigates the role of age and sex on the cardiovascular effects of 3.5-MHz pulsed ultrasound (US) in a rat model. Ultrasonic bursts of 2.0-MPa peak rarefactional pressure amplitude (equivalent to an in vitro spatial-peak temporal-peak intensity of ~270 W/cm² and a mechanical index of 1.1) were delivered in five consecutive 10-s intervals, one interval for each pulse repetition frequency (PRF) (6, 5, 4, 5, and 6 Hz; always the same order) for a total exposure duration of 50 consecutive seconds. Sixty F344 rats were split into 12 groups in a 3×2×2 factorial design (three ages, male versus female, and US application versus control). This study is the first study on US-induced cardiac effects that contains data across three age groups of rats (premenopause, fertile, and postmenopause) to mimic the fertile and nonfertile human window. US was applied transthoracically, while heart rate, stroke volume, ejection fraction, temperature, and other physiologic parameters were recorded at baseline and after exposure. Significant decreases in cardiac output compared to respective control groups were observed in multiple experimental groups, spanning both females and males. A negative chronotropic effect was observed in young male (~7%) and female (~16%) rats, in five-month-old male (~9%) and female (~15%) rats, and in old rats where the effect was not statistically significant. Younger groups and, to a lesser extent, lower weight groups generally had more significant effects. The pathophysiology of US-induced cardiovascular effects appears to be multifactorial and not strictly related to hormones, menopause, weight, sex, or age, individually.

Transient Receptor Potential Melastatin 2 Thermosensitive Neurons in the Preoptic Area Involved in Menopausal Hot Flashes in Ovariectomized Mice

INTRODUCTION

Menopausal hot flashes are related to hypothalamic preoptic area (POA) dysfunction. Thermosensitive transient receptor potential channels (ThermoTRPs) are involved in temperature sensing and regulation of thermosensitive neurons (TSNs) in the POA. Whether ThermoTRP-TSNs in the POA, particularly the non-noxious thermoreceptor, transient receptor potential melastatin 2 (TRPM2), are involved in the occurrence of hot flashes is still unclear.

METHODS

Twenty wild-type and 50 Trpm2-Cre adult female mice were randomly divided into sham (SHAM) and ovariectomy (OVX) groups. In the POA, ERα, ERβ, GPR30, TRPA1, TRPM8, TRPM2, and TRPV1 expression was detected by Western blot or/and quantitative real-time polymerase chain reaction and the number of TSNs expressing TRPM2 (TRPM2-TSNs) by immunofluorescence. Before and after TRPM2-TSN activation/inhibition, back (BST) and tail skin temperature (TST) and the proportion of glutamatergic and GABAergic neurons among TRPM2-TSNs were recorded.

RESULTS

Compared with SHAM, the expression of ERα, ERβ, TRPM2, and TRPM8 in the POA of the OVX group decreased, with a significantly larger change range for TRPM2 than TRPM8. In addition, the number of TRPM2-TSNs showing TRPA1, TRPM8, and TRPV1 expression in the OVX group decreased, and the proportion of glutamatergic and GABAergic neurons in TRPM2-TSNs decreased and increased, respectively. Meanwhile, BST and TST increased. After activating or inhibiting TRPM2-TSNs, the proportions of glutamatergic and GABAergic neurons in TRPM2-TSNs changed, along with the BST and TST.

CONCLUSION

In menopause, the abnormal quantity and function of TRPM2-TSNs in the POA is key for the development of hot flashes, characterized by an imbalance in heat dissipation and production due to the corresponding imbalance in glutamatergic and GABAergic neurons.

Effects of neurokinin 3 receptor antagonist fezolinetant on hot flash-like symptoms in ovariectomized rats

The majority of women experience vasomotor symptoms (VMS), such as hot flashes and night sweats, during the menopausal transition. Recent evidence strongly suggests a connection between neurokinin 3 (NK3) receptor signaling and VMS associated with menopause. The NK3 receptor antagonist fezolinetant is currently in phase 3 development for treatment of moderate to severe VMS associated with menopause. We investigated the pharmacological effects of repeated administration of fezolinetant on levels of sex hormones and gonadotropins, neuronal activity in the hypothalamus, and skin temperature as an index of hot flash-like symptoms in ovariectomized rats as a model of menopause. Ovariectomized rats exhibited several typical menopausal symptoms: hyperphagia, increased body weight, significantly decreased plasma estradiol levels, increased luteinizing hormone (LH) and follicle-stimulating hormone (FSH) levels, and significantly increased skin temperature. Increased c-Fos expression (an indirect marker of neuronal activity) in median preoptic nucleus (MnPO) hypothalamic neurons was also observed in ovariectomized rats. Repeated oral administration of fezolinetant (1-10 mg/kg, twice daily) for 1 week dose-dependently reduced plasma LH levels without affecting estradiol or FSH levels, inhibited the activation of MnPO neurons, and attenuated hot flash-like symptoms. In addition, fezolinetant dose-dependently reduced hyperphagia and weight gain in ovariectomized rats. These preclinical findings suggest that fezolinetant attenuates hot flash-like symptoms via inhibition of neuronal activity in the MnPO of ovariectomized rats and provides further support for the ongoing clinical development of fezolinetant for the treatment of VMS associated with menopause.

The Effects of Estrogens on Neural Circuits That Control Temperature

Declining and variable levels of estrogens around the time of menopause are associated with a suite of metabolic, vascular, and neuroendocrine changes. The archetypal adverse effects of perimenopause are vasomotor symptoms, which include hot flashes and night sweats. Although vasomotor symptoms are routinely treated with hormone therapy, the risks associated with these treatments encourage us to seek alternative treatment avenues. Understanding the mechanisms underlying the effects of estrogens on temperature regulation is a first step toward identifying novel therapeutic targets. Here we outline findings in rodents that reveal neural and molecular targets of estrogens within brain regions that control distinct components of temperature homeostasis. These insights suggest that estrogens may alter the function of multiple specialized neural circuits to coordinate the suite of changes after menopause. Thus, defining the precise cells and neural circuits that mediate the effects of estrogens on temperature has promise to identify strategies that would selectively counteract hot flashes or other negative side effects without the health risks that accompany systemic hormone therapies.

Long-term oral administration of a novel estrogen receptor beta agonist enhances memory and alleviates drug-induced vasodilation in young ovariectomized mice

Development of estrogen therapies targeting the β (ERβ) but not α (ERα) estrogen receptor is critically needed for the treatment of negative menopausal symptoms, as ERα activation increases health risks like cancer. Here, we determined the effects of long-term oral treatment with EGX358, a novel highly selective ERβ agonist, on memory, vasodilation, and affect in young ovariectomized mice. Mice were orally gavaged daily for 9 weeks with vehicle, 17β-estradiol (E₂), the ERβ agonist diarylpropionitrile (DPN), or EGX358 at doses that enhance memory when delivered acutely. Tail skin temperature was recorded as a proxy for vasodilation following injection of vehicle or senktide, a tachykinin receptor 3 agonist used to model hot flashes. Anxiety-like behavior was assessed in the open field (OF) and elevated plus maze (EPM), and depression-like behavior was measured in the tail suspension (TST) and forced swim tests (FST). Finally, memory was assessed in object recognition (OR) and object placement (OP) tasks. E₂, DPN, and EGX358 reduced senktide-mediated increases in tail skin temperature compared to vehicle. All three treatments also enhanced memory in the OR and OP tasks, whereas vehicle did not. Although E₂ increased time spent in the center of the OF, no other treatment effects were observed in the OF, EPM, TST, or FST. These data suggest that long-term ERβ activation can reduce hot flash-like symptoms and enhance spatial and object recognition memories in ovariectomized mice. Thus, the highly selective ERβ agonist EGX358 may be a promising avenue for reducing menopause-related hot flashes and memory dysfunction.

Estrogen receptor alpha in the brain mediates tamoxifen-induced changes in physiology in mice

Adjuvant tamoxifen therapy improves survival in breast cancer patients. Unfortunately, long-term treatment comes with side effects that impact health and quality of life, including hot flashes, changes in bone density, and fatigue. Partly due to a lack of proven animal models, the tissues and cells that mediate these negative side effects are unclear. Here, we show that mice undergoing tamoxifen treatment experience changes in temperature, bone, and movement. Single-cell RNA sequencing reveals that tamoxifen treatment induces widespread gene expression changes in the hypothalamus and preoptic area (hypothalamus-POA). These expression changes are dependent on estrogen receptor alpha (ERα), as conditional knockout of ERα in the hypothalamus-POA ablates or reverses tamoxifen-induced gene expression. Accordingly, ERα-deficient mice do not exhibit tamoxifen-induced changes in temperature, bone, or movement. These findings provide mechanistic insight into the effects of tamoxifen on the hypothalamus-POA and indicate that ERα mediates several physiological effects of tamoxifen treatment in mice.

A phase 2b, randomized, placebo-controlled, double-blind, dose-ranging study of the neurokinin 3 receptor antagonist fezolinetant for vasomotor symptoms associated with menopause

Objective:

Menopausal vasomotor symptoms (VMS) may result from altered thermoregulatory control in brain regions innervated by neurokinin 3 receptor-expressing neurons. This phase 2b study evaluated seven dosing regimens of fezolinetant, a selective neurokinin 3 receptor antagonist, as a nonhormone approach for the treatment of VMS.

Methods:

Menopausal women aged >40-65 years with moderate/severe VMS (≥50 episodes/wk) were randomized (double-blind) to fezolinetant 15, 30, 60, 90 mg BID or 30, 60, 120 mg QD, or placebo for 12 weeks. Primary outcomes were reduction in moderate/severe VMS frequency and severity ([number of moderate VMS × 2] + [number of severe VMS × 3]/total daily moderate/severe VMS) at weeks 4 and 12. Response (≥50% reduction in moderate/severe VMS frequency) was a key secondary outcome.

Results:

Of 352 treated participants, 287 completed the study. Fezolinetant reduced moderate/severe VMS frequency by −1.9 to −3.5/day at week 4 and −1.8 to −2.6/day at week 12 (all P < 0.05 vs placebo). Mean difference from placebo in VMS severity score was −0.4 to −1 at week 4 (all doses P < 0.05) and −0.2 to −0.6 at week 12 (P < 0.05 for 60 and 90 mg BID and 60 mg QD). Response (50% reduction) relative to placebo was achieved by 81.4% to 94.7% versus 58.5% of participants at end of treatment (all doses P < 0.05). Treatment-emergent adverse events were largely mild/moderate; no serious treatment-related treatment-emergent adverse events occurred.

Conclusions:

Fezolinetant is a well-tolerated, effective nonhormone therapy that rapidly reduces moderate/severe menopausal VMS.

Video Summary:http://links.lww.com/MENO/A572; video script available at http://links.lww.com/MENO/A573.

Estrogen-sensitive medial preoptic area neurons coordinate torpor in mice

Homeotherms maintain a stable internal body temperature despite changing environments. During energy deficiency, some species can cease to defend their body temperature and enter a hypothermic and hypometabolic state known as torpor. Recent advances have revealed the medial preoptic area (MPA) as a key site for the regulation of torpor in mice. The MPA is estrogen-sensitive and estrogens also have potent effects on both temperature and metabolism. Here, we demonstrate that estrogen-sensitive neurons in the MPA can coordinate hypothermia and hypometabolism in mice. Selectively activating estrogen-sensitive MPA neurons was sufficient to drive a coordinated depression of metabolic rate and body temperature similar to torpor, as measured by body temperature, physical activity, indirect calorimetry, heart rate, and brain activity. Inducing torpor with a prolonged fast revealed larger and more variable calcium transients from estrogen-sensitive MPA neurons during bouts of hypothermia. Finally, whereas selective ablation of estrogen-sensitive MPA neurons demonstrated that these neurons are required for the full expression of fasting-induced torpor in both female and male mice, their effects on thermoregulation and torpor bout initiation exhibit differences across sex. Together, these findings suggest a role for estrogen-sensitive MPA neurons in directing the thermoregulatory and metabolic responses to energy deficiency.

Influence of exogenous and endogenous estrogen on thermoregulatory responses to mild heat and the interaction with light and dark phases

The present study aimed to determine the influence of estradiol (E2) and the interaction with circadian phases on thermoregulatory responses to mild heat in female rats. Heat loss and production during 3-h exposure to the environment at an ambient temperature of 28-34 °C were assessed by measuring abdominal temperature (Tabd), tail skin temperature, and oxygen consumption in ovariectomized rats with and without E2 replacement (OVX + E2 and OVX, respectively) and in control rats in the proestrus (P) and diestrus (D) phases. In the light phase, Tabd remained unchanged in all groups. Tabd increased in the dark phase, but was lower in the OVX + E2 and P groups than in the OVX and D groups. Oxygen consumption decreased at 34 °C, but to a lesser extent in the OVX + E2 group than in the OVX group. These results suggest that E2 activates thermoregulation in mild heat in the dark phase.

The contribution of the mouse tail to thermoregulation is modest

Understanding mouse thermal physiology informs the usefulness of mice as models of human disease. It is widely assumed that the mouse tail contributes greatly to heat loss (as it does in rat), but this has not been quantitated. We studied C57BL/6J mice after tail amputation. Tailless mice housed at 22°C did not differ from littermate controls in body weight, lean or fat content, or energy expenditure. With acute changes in ambient temperature from 19 to 39°C, tailless and control mice demonstrated similar body temperatures (Tb), metabolic rates, and heat conductances and no difference in thermoneutral point. Treatment with prazosin, an α1-adrenergic antagonist and vasodilator, increased tail temperature in control mice by up to 4.8 ± 0.8°C. Comparing prazosin treatment in tailless and control mice suggested that the tail's contribution to total heat loss was a nonsignificant 3.4%. Major heat stress produced by treatment at 30°C with CL316243, a β3-adrenergic agonist, increased metabolic rate and Tb and, at a matched increase in metabolic rate, the tailless mice showed a 0.72 ± 0.14°C greater Tb increase and 7.6% lower whole body heat conductance. Thus, the mouse tail is a useful biomarker of vasodilation and thermoregulation, but in our experiments contributes only 5-8% of whole body heat dissipation, less than the 17% reported for rat. Heat dissipation through the tail is important under extreme scenarios such as pharmacological activation of brown adipose tissue; however, non-tail contributions to heat loss may have been underestimated in the mouse.

The hypertensive potential of estrogen: An untold story

Cardiovascular disease (CVD) is the major cause of morbidity and mortality worldwide. The implication of estrogen in this disease has been extensively studied. While the vast majority of published research argue for a cardioprotective role of estrogen in vascular inflammation such as in atherosclerosis, the role of estrogen in hypertension remains far from being resolved. The vasorelaxant effect of estrogen has already been well-established. However, emerging evidence supports a vasoconstrictive potential of this hormone. It has been proposed that the microenvironment dictates the effect of estrogen-induced type 1 nitric oxide synthase-1 (nNOS) on vasotone. Indeed, depending on nNOS product, nitric oxide or superoxide, estrogen can induce vasodilation or vasoconstriction, respectively. In this review, we discuss the evidence supporting the vasorelaxant effects of estrogen, and the molecular players involved. Furthermore, we shed light on recent reports revealing a vasoconstrictive role of estrogen, and speculate on the underlying signaling pathways. In addition, we identify certain factors that can account for the discrepant estrogenic effects. This review emphasizes a yin-yang role of estrogen in regulating blood pressure.

Gonadal hormones influence core body temperature during calorie restriction

During calorie restriction (CR), endotherms adjust several physiological processes including the decrease of core body temperature (T_b) and reduction of energy expenditure. We recently found that CR-induced hypothermia is regulated in a sex-dependent manner in mice with lowered central insulin-like growth factor receptor signaling. Here, we describe the contribution of sex hormones to CR-induced hypothermia in wild type C57BL6 mice by measuring T_b of female and male mice following bilateral gonadectomy and hormonal replacement. Specifically, we evaluated the effects of progesterone (P4), 17-ß estradiol (E2), a combination of both (P4 + E2) in females and of 5-α dihydrotestosterone (5-α DHT) in males. Gonadectomy resulted in an earlier and stronger CR-induced hypothermia in both sexes. These effects were fully antagonized in females by E2 replacement, but not by P4, which had only minor and partial effects when used alone and did not prevent the action of E2 during CR when both hormones were given in combination. 5-α-DHT had only minor and transient effects on preventing the reduction of T_b during CR on gonadectomized male mice. These findings indicate that gonadal hormones contribute to sex-specific regulation of T_b and energy expenditure when nutrient availability is scarce. Abbreviations: AL: ad libitum; ANOVA: analysis of variance; CR: calorie restriction; E2: 17-ß estradiol; GNX: gonadectomy or gonadectomized; IGF-1R: insulin-like growth factor 1 receptor; POA: preoptic area; P4: progesterone; RM: repeated measures; SD: standard deviation; SEM: standard error of mean; T_b: core body temperature; WT: wildtype; 5-α DHT: 5-α dihydrotestosterone.

Glutamatergic Neurokinin 3 Receptor Neurons in the Median Preoptic Nucleus Modulate Heat-Defense Pathways in Female Mice

We have proposed that arcuate neurons coexpressing kisspeptin, neurokinin B, and dynorphin (KNDy neurons) contribute to hot flushes via projections to neurokinin 3 receptor (NK3R)-expressing neurons in the median preoptic nucleus (MnPO). To characterize the thermoregulatory role of MnPO NK3R neurons in female mice, we ablated these neurons using injections of saporin toxin conjugated to a selective NK3R agonist. Loss of MnPO NK3R neurons increased the core temperature (TCORE) during the light phase, with the frequency distributions indicating a regulated shift in the balance point. The increase in TCORE in the ablated mice occurred despite changes in the ambient temperature and regardless of estrogen status. We next determined whether an acute increase in ambient temperature or higher TCORE would induce Fos in preoptic enhanced green fluorescent protein (EGFP)-immunoreactive neurons in Tacr3-EGFP mice. Fos activation was increased in the MnPO but no induction of Fos was found in NK3R (EGFP-immunoreactive) neurons. Thus, MnPO NK3R neurons are not activated by warm thermosensors in the skin or viscera and are not warm-sensitive neurons. Finally, RNAscope was used to determine whether Tacr3 (NK3R) mRNA was coexpressed with vesicular glutamate transporter 2 or vesicular γ-aminobutyric acid (GABA) transporter mRNA, markers of glutamatergic and GABAergic neurotransmission, respectively. In the MnPO, 94% of NK3R neurons were glutamatergic, but in the adjacent medial preoptic area, 97% of NK3R neurons were GABAergic. Thus, NK3R neurons in the MnPO are glutamatergic and play a role in reducing TCORE but are not activated by warm thermal stimuli (internal or external). These findings suggest that KNDy neurons modulate thermosensory pathways for heat defense indirectly via a subpopulation of glutamatergic MnPO neurons that express NK3R.

Neurokinin 3 Receptor Antagonism: A Novel Treatment for Menopausal Hot Flushes

Menopause is associated with significant symptomatic burden, with approximately two-thirds of postmenopausal women suffering from vasomotor symptoms, hot flushes, and night sweats. The mainstay of treatment for hot flushes continues to be hormone replacement therapy. However, as hormone replacement therapy is contraindicated in some cases, alternative, efficacious treatment options are also required. Hot flushes are thought to arise as a result of significant changes in the neuroendocrine circuitry underpinning the reproductive axis during menopause. This includes reduced circulating ovarian oestrogens, hypersecretion of gonadotropins, and increased expression of kisspeptin and neurokinin B (NKB) within the infundibular nucleus of the hypothalamus. In recent years, NKB, predominantly acting via the neurokinin 3 receptor (NK3R), has emerged as an important player in the development of menopausal hot flushes. Antagonism of NK3R has garnered much interest as a novel therapeutic target to help ameliorate hot flush symptoms. Improvements in hot flush frequency, severity, and quality of life have been demonstrated in a number of clinical trials using novel NK3R antagonists in postmenopausal women. Within this review, we will explore the growing body of evidence supporting antagonism of NK3R as a potentially promising treatment for menopausal hot flushes.